9 research outputs found
Two-center interference and ellipticity in high-order harmonic generation from H
We present a theoretical investigation into the two-center interference in
aligned H. The influence of the laser field on the recombination step is
investigated by comparing laser-induced harmonic generation with harmonic
generation from field-free collisions of Gaussian wave packets with the core.
We find that for different Gaussian wave packets colliding with the molecule,
the interference minimum occurs at the same alignment angle. The same result is
obtained for the laser-induced spectrum when only a single electronic
trajectory per harmonic contributes. When multiple electronic trajectories
contribute, we find an effect on the minimum position because the interference
between short and long trajectories is alignment-dependent. The two-center
interference and the influence of the Coulombic potential are clearly seen not
only in the harmonic intensity and phase but also in the polarization direction
and ellipticity. We observe significant ellipticity of the emitted radiation
around the two-center interference minimum.Comment: 10 pages, 15 figures; v2: clearer figures and other small changes;
v3: small correction
Control of recollision wave packets for molecular orbital tomography using short laser pulses
The tomographic imaging of arbitrary molecular orbitals via high-order
harmonic generation requires that electrons recollide from one direction only.
Within a semi-classical model, we show that extremely short phase-stabilized
laser pulses offer control over the momentum distribution of the returning
electrons. By adjusting the carrier-envelope phase, recollisions can be forced
to occur from mainly one side, while retaining a broad energy spectrum. The
signatures of the semi-classical distributions are observed in harmonic spectra
obtained by numerical solution of the time-dependent Schr\"{o}dinger equation.Comment: 8 pages, 4 figures; v2: Added some extra clarifications; v3: minor
grammatical change
Molecular imaging using high-order harmonic generation and above-threshold ionization
Accurate molecular imaging via high-order harmonic generation relies on
comparing the harmonic emission from a molecule and an adequate reference
system. However, an ideal reference atom with the same ionization properties as
the molecule does not always exist. We show that for suitably designed, very
short laser pulses, a one-to-one mapping between high-order harmonic
frequencies and electron momenta in above-threshold ionization exists.
Comparing molecular and atomic momentum distributions then provides the
electron return amplitude in the molecule for every harmonic frequency. We show
that the method retrieves the molecular recombination transition moments highly
accurately, even with suboptimal reference atoms.Comment: 5 pages, 4 figure
Emission times in high-order harmonic generation
We calculate the emission times of the radiation in high-order harmonic generation using the Gabor transform of numerical data obtained from solving the time-dependent Schrödinger equation in one, two, and three dimensions. Both atomic and molecular systems, including nuclear motion, are investigated. Lewenstein model calculations are used to gauge the performance of the Gabor method. The resulting emission times are compared against the classical simple man's model as well as against the more accurate quantum orbit model based on complex trajectories. The influence of the range of the binding potential (long or short) on the level of agreement is assessed. Our analysis reveals that the short-trajectory harmonics are emitted slightly earlier than predicted by the quantum orbit model. This partially explains recent experimental observations for atoms and molecules. Furthermore, we observe a distinct signature of two-center interference in the emission times for H2 and D2. © 2010 The American Physical Society
Molecular Orbital Tomography using Short Laser Pulses
Recently, a method to image molecular electronic wave functions using high
harmonic generation (HHG) was introduced by Itatani \textit{et al.\} [Nature
{\textbf{432}}, 876 (2004)]. We show that, while the tomographic reconstruction
of general orbitals with arbitrary symmetry cannot be performed with long laser
pulses, this becomes possible when extremely short pulses are used. An
alternative reconstruction equation based on momentum matrix elements, rather
than on dipole matrix elements, is proposed. We present simulations of the
procedure for 2D model systems based on numerical solutions of the
time-dependent Schr\"{o}dinger equation, and present results from further
post-processing of the reconstructed orbitals.Comment: 5 pages, 2 figure
Tomographic imaging of molecular orbitals in length and velocity form
Max-Planck-Institut für Kernphysik, Heidelber